There is a general desire in optical transmission links to increase the performance capability of the transmission link, for example the data throughput in the transmission.
Optical fibers are particularly available for high-capacity, line bound transmission links. It is known that the regions of low attenuation in these fibers at the optical wavelength 1.3 .mu.m and 1.55 .mu.m, which are referred to as the optical windows, together correspond to a transmission band width of more than 30,000 GHz. The desire is to make the optimally large part of this band width employable or usable.
Various methods are known for increasing the performance capabilities of a transmission link.
The data throughput can first be increased by increasing the data rate by electrical or optical time-division multiplexing of the data signals. On the other hand, a limit thereby occurs with respect to the prevailing data rate in the electronics or, respectively, the optoelectronics, and this limit currently amounts to a maximum of 10 Gbit/s and perhaps in the future a range of 50 through 100 Gbit/s per second. On the other hand, the distance that can be bridged decreases with the increasing data rate due to the decreasing receiver sensitivity or due to an increasing dispersion-caused transit time effects. The first limitation can be largely cancelled by the employment of optical amplifiers or repeaters. The second, however, is more difficult to avoid and an improvement can be achieved by employing specific, low dispersion fibers. The employment of optical solitons for the transmission is significantly more effective, but more involved as well, for example see M. Nakazawa et al "10 Gbits/s-1200 km single pass soliton data transmission using Erbium-doped fiber amplifiers", Post Deadline Paper, PD 11, OFC '92, Feb. 2-7, 1992, San Jose, Calif. The dispersion effects are compensated here by non-linear effects on the fiber.
An especially elegant way of enhancing the performance of the transmission link with respect to the data throughput is the optical multi-channel technique in combination with a time-division multiplex technique. The performance capabilities of this technique, however, is limited, particularly by the channel cross-talk due to non-linear effects of the transmission link.
In optical multi-channel technique in the form of an optical wavelength-division multiplex, the optical signals to be transmitted are transmitted on many neighboring, optical carrier wavelengths.
The performance capability of such a transmission system, however, is limited by a number of parameters. These include the use of a part of the optical window, which is particularly limited due to the availability or the variable frequency range of the optical transmitters. The minimum channel or carrier wavelength spacing derived from the selectivity or tuning precision of the available optical receiver, for example, the resolution of a demultiplexer or of an optical given direct reception or of the intermediate frequency filter band with given heterodyne reception, from the band width of the modulated optical data channel as well as on the basis of channel cross-talk due to non-linear effects on the transmission path.
It is precisely the channel cross-talk due to non-linear effects that represents a great restriction in the multi-channel system given small channel spacings and a high number of channels, particularly when the high transmission power must be utilized because great distances are to be bridged with a high data rate per channel. It is particularly the three-wave mixing, which is often referred to as a four-wave mixing, and the raman scatter that dominates the non-linear interaction in multi-channel systems. A detailed calculation of the influence of these effects on transmission systems with optical heterodyne reception can be found, for example, in an article by Andrew R. Chraplyvy, "Limitations on Light-Wave Communications Imposed by Optical-Fiber Non-linearities", J. Lightwave Techn., Vol. 8 (1990), pages 1548-1557.